The invention relates in general to magnetic field sensors and more particularly to magnetic field sensors requiring the use of a magnetic field for functions such as domain setting, calibration, establishing a bias or offset magnetic field, determining a sensor transfer characteristic, or other functions requiring a magnetic field.
The invention has application to a variety of magnetic sensors or transducers. These include, but are not limited to magnetoresistive transducers, magnetodiodes, magnetotransistors and Hall effect transducers.
The domain setting function is important for magnetoresistive transducers. Magnetoresistive transducers are typically defined on a layer above the integrated circuit wafer or on a glass slide. Magnetoresistive transducers are composed of long thin strips of a magnetoresistive film such as permalloy. With a current running along the length of the film, the magnetization of the film generally forms an angle with the current, and the resistance of the film depends on this angle. When the magnetization of the film is parallel to the current, the resistance is at a maximum, and when it is perpendicular to the current, the resistance has its minimum value.
The magnetization in these films must be set in a single domain state before it is ready to sense magnetic fields. Although there may be situations where a magnetoresistive transducer can sense magnetic fields even if it is in a multiple domain state, for purposes of repeatability, it is essential that the magnetization of a magnetoresistive transducer magnetic sensing element be in a single domain state. In certain designs of transducers, this domain setting can be done using a large external field. However, this setting approach is not practical when the magnetoresistive transducer has already been packaged in a system. In addition, in many situations there are several transducers within one package with different transducers requiring setting in different directions. While one potential solution is to wrap individual coils around each transducer; the use of individual coils is expensive and it is difficult to generate large fields by using small coils. Another situation frequently arising is when different legs of a bridge circuit, for example a Wheatstone bridge circuit fabricated on a single chip, have to have their magnetization set in different directions.
The testing, setup, or calibration of magnetic field sensing devices represents a second important area. The function needed is the ability to produce a known magnetic field at the magnetic field sensor. This known field and the ability to vary the field allows one to measure the response of the magnetic field sensor and perform set up, sensitivity and calibration operations. In the past these operations have been performed by producing the known magnetic field by using a pair of Helmholtz coils or a solenoid coil. These methods require the use of external field producing coils and cannot be used when multiple magnetic field sensors have been placed in a single package. In addition, the external coil approach does not permit the magnetic field sensor to be tested and calibrated in its working environment.
Thus a need exists for a simple self contained device to provide a magnetic field for setting and resetting the magnetic domains in a sensor and for producing a known magnetic field for testing, set up, and calibration of a magnetic field sensor.